In recent years, there is more need for improvement in automobile fuel efficiency, as exemplified by the establishment of a new target for automobile fuel efficiency improvement and the introduction of tax privileges for low fuel consumption vehicles, as measures for reducing carbon dioxide emissions aimed at the prevention of global warming. The weight reduction of an automobile is effective as a means for improving fuel efficiency and, from the viewpoint of such weight reduction, a material having a higher tensile strength is strongly demanded. On the contrary, generally speaking, the press formability of a material deteriorates as the strength of the material increases. Therefore, the development of a steel sheet satisfying both press formability and high strength is desired in order to attain the weight reduction of such a member. There are an elongation measured by a tensile test, an n-value and an r-value as indices of formability. Nowadays, the simplification of a press process by integral forming is a current issue and therefore, among those indices, a large n-value that corresponds to a uniform elongation is being regarded as an important index.
Then, a hot-dip galvanized steel sheet is also required to have a higher tensile strength. In order to attain both a higher tensile strength and workability, it is necessary to add elements such as Si, Mn and Al. However, when such Si, Mn and Al are contained as components of a steel sheet, there arises a problem in that oxides that have poor wettability with a plating layer are formed during annealing in a reducing atmosphere, incrassate on the surface of the steel sheet and deteriorate the plating performance of the steel sheet. In other words, the elements such as Si, Mn and Al have a high oxidizability and for that reason they are preferentially oxidized in a reducing atmosphere, incrassate on the surface of a steel sheet, deteriorate plating wettability, generate so-called non-plated portions, and thus result in the deterioration of plating appearance.
In this light, in order to produce a high-strength hot-dip galvanized steel sheet, it is essential to suppress the formation of oxides containing Si, Mn, Al etc. as mentioned above. From this point of view, various technologies have so far been proposed. For example, Japanese Unexamined Patent Publication No. H7-34210 proposes the method wherein a steel sheet is heated to 400° C. to 650° C. for oxidizing Fe in an atmosphere having an oxygen concentration in the range from 0.1 to 100% in the preheating zone of an annealing furnace of oxidization-reduction type equipment and thereafter subjected to ordinary reduction annealing and hot-dip galvanizing treatment. In this method however, since the effect depends on the Si content in a steel sheet, it is not said that plating performance is sufficient in the case of a steel sheet having a high Si content. Here, though there may sometimes be a state where non-plated portions are not formed if it is immediately after the formation of a plating layer, since the plating adhesiveness is insufficient, the problems of plating exfoliation and others may sometimes occur when various processing is applied to a hot-dip galvanized steel sheet after the formation of a plating layer. In other words, though Si addition is a requirement essential for the improvement of the workability of a steel sheet, such an amount of Si as necessary for the improvement of the workability cannot be added from the restrictions for securing plating performance by the aforementioned technology and therefore the technology cannot be a fundamental solution. Further, another problem of the technology is that the technology cannot be used in equipment having the capability of only reduction annealing since this method is applicable to only oxidization-reduction type equipment.
Meanwhile, though non-plated portions can also be avoided by applying reduction annealing and hot-dip plating in the state of forming Fe, Ni etc. on the surface of a steel sheet by electroplating beforehand, such a method requires additional electroplating equipment and causes an additional problem of the increase of the number of the processes and resultant cost increase.
Further, Japanese Patent No. 3126911 proposes the method wherein plating adhesiveness is improved by forming oxides at the grain boundaries of a steel sheet containing Si and Mn through a high temperature coiling at the stage of hot rolling. However, since this method requires a high temperature coiling at the stage of hot rolling, the problems thereof are: that pickling load after hot rolling increases as a result of the increase of oxidized scales, thus productivity deteriorates and resultantly the cost increases; that the surface appearance of the steel sheet deteriorates because grain boundary oxidization is formed on the surface of the steel sheet; and that the fatigue strength deteriorates with the grain boundary oxidized portions functioning as the origin.
Furthermore, for example, Japanese Unexamined Patent Publication No. 2001-131693 discloses the method wherein a steel sheet is annealed firstly in a reducing atmosphere having a dew point of 0° C. or lower, thereafter oxides on the surface of the steel sheet are removed by pickling, and subsequently the steel sheet is annealed secondly in a reducing atmosphere having a dew point of −20° C. or lower and then subjected to hot-dip plating. However, the problem of the method is that annealing must be applied twice and thus the production cost increases. Yet further, Japanese Unexamined Patent Publication No. 2002-47547 discloses the method wherein internal oxidization is formed in the surface layer of a steel sheet by applying heat treatment after hot rolling while black skin scales are attached to the steel sheet. However, the problem of the method is that a process for black skin annealing must be added and thus the production cost also increases.
Moreover, WO 00/50658 proposes the technology wherein Ni is added in an appropriate amount to a steel containing Si and Al. However, the problem caused by the technology is that, when the technology is intended to be applied to practical production, the plating performance varies with a reduction annealing furnace only and resultantly a good steel sheet cannot be produced stably.
In the meantime, a hot-rolled steel sheet and a cold-rolled steel sheet obtained by utilizing the transformation-induced plasticity of retained austenite contained in the steel are developed. Those are the steel sheets, each of which contains retained austenite in the metallographic structure through heat treatment, that is characterized by: containing only about 0.07 to 0.4% C, about 0.3 to 2.0% Si and about 0.2 to 2.5% Mn as basic alloying elements without containing expensive alloying elements; and applying bainite transformation in the temperature range nearly from 300° C. to 450° C. after annealing in a dual phase zone. For example, Japanese Unexamined Patent Publication Nos. H1-230715 and H2-217425 disclose such steel sheets. As such steel sheets, not only a cold-rolled steel sheet is produced through continuous annealing but also it is disclosed that a hot-rolled steel sheet can also be obtained by controlling the cooling on run-out tables and a coiling temperature in Japanese Unexamined Patent Publication No. H1-79345, for example.
The trend of applying plating to automobile members is growing with the aim of improving corrosion resistance and appearance in conformity with the trend of a higher-grade automobile and galvanized steel sheets are presently used for a variety of members excluding specific members mounted in the interior of an automobile. Therefore, it is effective from the viewpoint of corrosion resistance to use a steel sheet subjected to hot-dip galvanizing or alloying hot-dip galvanizing wherein alloying treatment is applied after hot-dip galvanizing as such a steel sheet. However, in the case of a steel sheet having high Si and Al contents among such high-strength steel sheets, there is the problem in that an oxide film tends to form on the surface of the steel sheet, therefore fine non-plated portions are generated at the time of hot-dip galvanizing, and resultantly the plating performance deteriorates at the portions processed after alloying. Therefore, it is the present situation that a high-strength high-ductility alloyed hot-dip galvanized steel sheet of high Si and Al type, the steel sheet being excellent in corrosion resistance and plating performance at processed portions, is not practically applied.
In the case of a steel sheet disclosed in Japanese Unexamined Patent Publication Nos. H1-230715 and H2-217425 for example, since Si is added by 0.3 to 2.0% and retained austenite is secured by utilizing the unique bainite transformation, an intended metallographic structure cannot be obtained and the strength and elongation deviate from the target ranges unless the cooling after annealing in the dual phase coexisting temperature range and the retention of the steel sheet in the temperature range nearly from 300° C. to 450° C. are extremely strictly controlled. Such a heat history can be realized industrially in continuous annealing equipment, run-out tables after hot rolling and a coiling process. In this case, when the temperature range is from 450° C. to 600° C., since the transformation of austenite is completed soon, such control as to particularly shorten the time duration where a steel sheet is retained in the temperature range from 450° C. to 600° C. is required. Even when the temperature range is from 350° C. to 450° C., since the metallographic structure varies considerably in accordance with the retention time, only poor strength and elongation are obtained in the case of deviating from prescribed conditions. Further, the problem here is that, since the retention time in the temperature range from 450° C. to 600° C. is long and Si that deteriorates plating performance is contained as an alloying element, it is impossible to produce a plated steel sheet through hot-dip plating equipment, the surface corrosion resistance is inferior, and thus a wide range of industrial application is hindered.
In order to solve the aforementioned problems, for example, Japanese Unexamined Patent Publication Nos. H5-247586 and H6-145788 disclose a steel sheet having the plating performance which is improved by regulating an Si concentration. In this method, retained austenite is formed by adding Al instead of Si. However, the problem of the method is that, since Al, like Si, is also more likely to be oxidized than Fe, Al and Si tend to incrassate and form an oxide film on the surface of a steel sheet and sufficient plating performance is not obtained. Further, Japanese Unexamined Patent Publication No. H5-70886 discloses the technology wherein plating wettability is improved by adding Ni. However, the method does not disclose the relationship between Ni and the group of Si and Al that deteriorate plating wettability.
Furthermore, for example, Japanese Unexamined Patent Publication Nos. H4-333552 and H4-346644 disclose the method wherein a steel sheet is subjected to rapid low temperature heating after Ni preplating, hot-dip galvanizing and successively alloying treatment as an alloying hot-dip plating method of a high Si type high-strength steel sheet. However, the problem of the method is that new equipment is required because Ni preplating is essential. Further, this method neither makes retained austenite remain in the final structure nor refers to a means to do so.
Yet further, for example, Japanese Unexamined Patent Publication No. 2002-234129 discloses the method wherein good properties are obtained by adding Cu, Ni and Mo to a steel sheet containing Si and Al. It says that, in the method, good plating performance and material properties can be obtained by properly adjusting the balance between the total amount of Si and Mn and the total amount of Cu, Ni and Mo. However, according to our investigation, a problem of the method is that the patent can not always secure good plating performance when Si is contained since the plating performance of a steel containing Si and Mn is dominated by the amount of Al. Further, another problem thereof is that the method is only applicable to a steel sheet having such relatively low strength as in the range from 440 to 640 MPa in tensile strength.
Moreover, the present inventors propose in PCT Patent Publication WO 00/50658 the technology wherein an appropriate amount of Ni is added to a steel containing Si and Al. However, the problem of the technology is that the quality of a material obtained by this method varies due to the dispersion of an alloying temperature in an attempt to produce an alloyed hot-dip galvanized steel sheet.